Fingermice are increasingly being used on electronic devices, such as mobile phones, keyboards, remote controls, laptops and tablet computers, as a way for providing a scrolling input. A fingermouse typically comprises a light source, an image sensor, and a mousing surface. Radiation is emitted by the light source towards the mousing surface. A user slides a finger across the mousing surface, and the sensor images light reflected by ridges and valleys on the fingerprint of the finger. By comparing subsequent images of the fingerprint captured by the sensor, motion vectors can be extracted. These motion vectors can then be translated into a scrolling input, for example, by a processor external to the image sensor.
As it is necessary for the mousing surface to be accessible, it usually faces the user during normal operation. Therefore, as there is typically a line of sight between the mousing surface and the image sensor, ambient light (e.g. sunlight) may impinge on the image sensor. If the ambient illumination is constant, filtering can reduce its effects. However, moving objects above the mousing surface (e.g. a user's fingers, hands, arms or leaves on a tree) and their shadows can cause changes in the image captured by the image sensor, ultimately causing unwanted scrolling inputs. Similarly, even if the objects above the mousing surface remain substantially still, movement of the device in which the image sensor is housed (e.g. mobile phone) may cause the image to move across the sensor, again causing unwanted scrolling inputs.
One possible solution to reducing unwanted scrolling inputs is to decrease the depth of focus of the imaging optics (e.g. by increasing the aperture or decreasing F number). This reduces the region around the fingermouse in which objects can be captured by the image sensor in sharp focus. However, the amount of blurring is not typically sufficient to prevent unwanted scrolling inputs. Decreasing the depth of focus also requires increased manufacturing tolerances, which increases costs. In addition, increasing the aperture of the imaging optics would require a higher quality and more costly imaging system.
Typically, fingermouse navigation algorithms may require images captured by the sensor to be processed, and a metric for the “number of features” is generated from the image. If this metric is higher than a predetermined threshold, the motion detection algorithm is enabled; otherwise, the motion detection algorithm is disabled. Thus, another possible solution to the problem of unwanted scrolling inputs is to increase the predetermined threshold required to enable the motion detection algorithm. However, this may cause the fingermouse to be unreliable, particularly for users with less well defined fingerprints (e.g. windsurfers). Thus, it is desirable to develop an improved fingermouse that may reduce unwanted scrolling inputs.